The present invention relates to a catalyst for producing a polyester and a process for producing the polyester by using the same. More particularly, the present invention relates to a catalyst for producing a polyester, comprising a specific titanium compound and a phosphorus compound, and a process for producing the polyester having a good color tone (b value), and thus not needing an addition of a cobalt compound to regulate the color tone, by using the catalyst.
The polyesters, particularly, polyethylene terephthalate, polyethylene naphthalate, polytrimethylene terephthalate and polytetramethylene terephthalate, have excellent mechanical, physical and chemical performances and thus are widely utilized for various fibers, films and other shaped articles.
For example, polyethylene terephthalate is usually produced by preparing an ethyleneglycol ester of terephthalic acid and/or oligomers of the ester by a direct esterification reaction of terephthalic acid with ethyleneglycol, or an interesterification reaction of a lower dialkyl ester of terephthalic acid, for example, dimethyl terephthalate, with ethyleneglycol, or a reaction of terephthalic acid with ethyleneoxide, and then by polymerizing ethyleneglycol ester of terephthalic acid and/or oligomer of the ester in the presence of a polymerization catalyst under a reduced pressure at an elevated temperature, until the polymerization product obtains a target degree of polymerization. Also, the polyethylene naphthalate, polytrimethylene terephthalate and polytetramethylene terephthalate can be produced by a process similar to that mentioned above.
It is well known that the reaction rate of the polymerization reaction and the quality of the resultant polyester are greatly influenced by the type of the catalyst for the polymerization reaction step. As a polymerization catalyst for the polyethylene terephthalate, antimony compounds are most widely employed, because they exhibit excellent catalytic property for polymerization, and the resultant polyester has a good color tone.
However, when the animony compounds are employed as a polymerization catalyst, the resultant polyester is disadvantageous in that when a melt-spinning procedure of the resultant polyester is continuously carried out for a long period, a foreign substance is cohered and accumulated around the melt-spinning orifice (which foreign substance will be referred to as an orifice foreign substance hereinafter), and causes the stream of the polymer melt extruded through the orifice to be bent (which will be referred to as a bending phenomenon of polymer melt stream hereinafter), and the bending phenomenon causes formation of fluffs and breakage of melt spun filaments to occur during the melt-spinning and drawing procedures. Namely, the shaping property of the polyester is degraded by the antimony compound catalyst.
As a polymerization catalyst other than the antimony compounds, the utilization of titanium compounds, for example, titanium tetrabutoxide is suggested. When the titanium compound as mentioned above is employed, the problem of accumulation of the orifice foreign substance can be solved. However, this catalyst causes a new disadvantage in that the resultant polyester per se is colored yellowish and exhibits a poor stability to heat-melting.
To solve the above-mentioned coloring problem, generally, a cobalt compound is mixed into the polyester to erase the yellowish color. It is true that the color tone (apparent whiteness) of the polyester can be improved by mixing the cobalt compound into the polyester. However, the mixed cobalt compound causes the stability of the polyester to heat melting to be decreased and thus the resultant polyester to be easily decomposed.
As a titanium compound for catalyst for the production of the polyester, it is known to employ titanium hydroxide in Japanese Examined Patent Publication No. 48-2,229, xcex1-titanic acid in Japanese Examined Patent Publication No. 47-26,597.
However, in the former catalyst the titanium hydroxide is difficult to pulverize and in the latter catalyst, xcex1-titanic acid is easily modified and is difficult to store and handle. Thus, both the catalysts are not suitable for practical industrial use, and also the catalysts are seldom produce a polyester having a good color tone (b value).
Also, Japanese Examined Patent Publication No. 59-46,258 discloses that a product obtained by a reaction of a titanium compound with trimellitic acid is used as a catalyst for producing the polyester, and Japanese Unexamined Patent Publication No. 58-38,722 discloses that a product of titanium compound with a phosphite ester is usable as a catalyst for producing the polyester. It is true that both the processes enable the stability of the polyester for melting heat to be enhanced to some extent. However, the color tone (colorlessness) of the resultant polyester is unsatisfactory. Thus, the color tone (b value) of the polyester must be further improved.
Further, Japanese Unexamined Patent Publication No. 7-138,354 discloses employment of a complex of a titanium compound with a phosphorus compound as a catalyst for producing the polyester. This process enables the stability of the resultant polyester to melting heat to some extent. However, the color tone of the resultant polyester is unsatisfactory.
Furthermore, when the phosphorus-containing catalyst as mentioned above is employed, the catalyst per se is usually retained, as a foreign substance, in the resultant polyester polymer. Thus, this problem must be solved.
An object of the present invention to provide a catalyst for producing a polyester having a good color tone (b value), a low content of a foreign substance and an excellent melting heat stability, and a process for producing a polyester by using the same.
The above-mentioned object can be attained by the catalyst of the present invention for producing a polyester and the process of the present invention for producing a polyester by using the catalyst. The catalyst of the present invention for producing the polyester comprises a reaction product of
(A) a titanium compound component comprising at least one member selected from the group consisting of:
titanium compounds (1) represented by the general formula (I): 
xe2x80x83in which formula (I), R1 represents an alkyl group having 1 to 10 carbon atoms and p represents an integer of 1 to 3, and
titanium compounds (2) produced by reacting the titanium compounds (1) of the formula (I) with aromatic polycarboxylic acids represented by the general formula (II): 
xe2x80x83in which formula (II), n represents an integer of 2 to 4, or anhydrides of the aromatic polycarboxylic acids of the formula (II), with
(B) a phosphorus compound component comprising at least one member selected from the phosphorus compounds (3) of the general formula (III): 
xe2x80x83in which formula (III), m represents an integer of 1 or 2, q represents an integer of 0 or 1, the sum of m and q, (m+q), is 1 or 2, R2 represents an unsubstituted or substituted aryl group having 6 to 20 carbon atoms or alkyl group having 1 to 20 carbon atoms and when m represents 2, the two R2 groups are the same as each other or different from each other.
The process of the present invention for producing a polyester comprises polymerizing a polymerization-starting material comprising at least one member selected from the group consisting of alkyleneglycol esters of aromatic difunctional carboxylic acids and oligomers thereof in the presence of a catalyst, wherein the catalyst comprises a reaction product of
(A) a titanium compound component comprising at least one member selected from the group consisting of:
titanium compounds (1) represented by the general formula (I): 
xe2x80x83in which formula (I), R1 represents an alkyl group having 1 to 10 carbon atoms and p represents an integer of 1 to 3, and
titanium compounds (2) produced by reacting the titanium compounds (1) of the formula (I) with aromatic polycarboxylic acids represented by the general formula (II): 
xe2x80x83in which formula (II), n represents an integer of 2 to 4, or anhydrides of the aromatic polycarboxylic acids of the formula (II), with
(B) a phosphorus compound component comprising at least one member selected from the phosphorus compounds (3) of the general formula (III): 
xe2x80x83in which formula (III), m represents an integer of 1 or 2, q represents an integer of 0 or 1, the sum of m and q, (m+q), is 1 or 2, R2 represents an unsubstituted or substituted aryl group having 6 to 20 carbon atoms or alkyl group having 1 to 20 carbon atoms and when m represent 2, the two R2 groups are the same as each other or different from each other; and
the amount of the catalyst in terms of molar amount in millimole of titanium atoms contained in the catalyst corresponds to 10 to 40% of the amount in millimole of the aromatic difunctional carboxlic acids contained in the polymerization-starting material.
The catalyst of the present invention for producing a polyester comprises a reaction product of a titanium compound component (A) with a phosphorus compound component (B) which will be explained in detail hereinafter.
The titanium compound component (A) usable for the catalyst of the present invention comprises at least one member selected from the group consisting of titanium compounds (1) represented by the following general formula (I): 
in which formula (I), R1 represents an alkyl group having 2 to 10 carbon atoms, preferably 3 to 6 carbon atoms, p represents an integer of 1 to 3, preferably 1 to 2; and titanium compounds (2) obtained by reacting the titanium compounds (1) of the general formula (I) with aromatic polycarboxylic acids represented by the following general formula (II): 
in which formula (II), n represents an integer of 2 to 4, preferably 2 to 3, or anhydrides of the aromatic polycarboxylic acid.
The phosphorus compound component (B) usable for the catalyst of the present invention comprises at least one member selected from the phosphorus compounds (3) represented by the following general formula (III): 
in which formula (III), m represents an integer of 1 or 2, q represents an integer of 0 or 1, the sum of m and q is 1 or 2, R2 represents an unsubstituted or substituted aryl group having 6 to 20 carbon atoms, preferably 6 to 10 carbon atoms, or alkyl group having 1 to 20 carbon atoms, preferably 2 to 12 carbon atoms, and when m represents an integer of 2, the two R2 groups may be the same as each other or different from each other.
In the reaction product of the titanium compound component (A) with the phosphorus compound component (B), usable as a catalyst of the present invention for producing the polyester, preferably, in the reaction product of the titanium compound component (A) with the phosphorus compound component (B), the ratio mTi/mp of the amount of the titanium compound component (A) in terms of a molar amount mTi of titanium atoms to the amount of the phosphorus compound component (B) in terms of a molar amount mp of phosphorus atoms is in the range of from 1:1 to 1:4, more preferably 1:1.5 to 1:2.5.
The phrase xe2x80x9can amount (mTi) of the titanium compound component (A) in terms of a molar amount of titanium atomsxe2x80x9d refers to as a total sum of products of molar amounts of individual titanium compounds and the numbers of the titanium atoms per molecule contained in individual titanium compounds.
Also, the phrase xe2x80x9can amount (mp) of the phosphorus compound component (B) in terms of a molar amount of phosphorus atomsxe2x80x9d refers to a total sum of products of molar amounts of individual phosphorus compounds and the numbers of the phosphorus atoms per molecule contained in the individual phosphorus compounds. The phosphorus compounds of the formula (III) contain only one phosphorus atoms per molecule of the individual phosphorus compounds. Thus, in this case, the amount of each phosphorus compound in terms of a molar amount of phosphorus atoms is the same as the real molar amount of each phosphorus compound.
When the reaction molar ratio mti/mp is more than 1:1, namely when the amount of the titanium compound component (A) is too much, the resultant catalyst may cause the polyester produced by using the catalyst to exhibit an unsatisfactory color tone (apparent whiteness) and an insufficient heat resistance. Also, when the reaction molar ratio mti/mp is less than 1:4, namely when the amount of the titanium compound component (A) is too small, the resultant catalyst may exhibit an insufficient catalytic activity for the production of the polyester.
The titanium compounds (1) of the general formula (I) usable for the titanium compound component (A) include, titanium tetraalkoxides, for example, titanium tetrabutoxide, titanium tetraisopropoxide, titanium tetrapropoxide, and titanium tetraethoxide; and alkyl titnates, for example, octaalkyl trititanates and hexaalkyl dititanates. Among them, the titanium tetraalkoxides which have a high reactivity to the phosphorus compounds usable for the present invention are preferably employed. Particularly, titanium tetrabutoxide is more preferably employed for the present invention.
The titanium compounds (2) usable for the titanium compound component (A) is produced by reacting the titanium compounds (1) of the general formula (I) with aromatic polycarboxilic acids of the general formula (III) or anhydrides thereof. The aromatic polycarboxylic acids of the general formula (II) and the anhydrides thereof are preferably selected from phthalic acid, trimellitic acid, hemimellitic acid and pyromellitic acid and anhydrides of the above-mentioned acids. Particularly, trimellitic anhydride, which has a high reactivity to the titanium compounds (1) and enables the resultant polymerization product to exhibit a high affinity with the resultant polyester, is preferably employed.
The reaction of the titanium compounds (1) with the aromatic polycarboxylic acids or the anhydrides thereof can be carried out by mixing the aromatic polycarboxylic acids or anhydrides thereof into a solvent to cause a portion or all of the polycarboxylic acids or the anhydrides thereof to be solved in the solvent; adding dropwise the titanium compound (1) into the mixture; and heating the reaction mixture at a temperature of 0 to 200xc2x0 C. for 30 minutes or more, preferably at 30 to 150xc2x0 C. for 40 to 90 minutes.
In the reaction, there is no specific restriction to the reaction pressure, and the reaction can be sufficiently effected under the ambient atmospheric pressure. The solvent for the above-mentioned reaction may be selected from those capable of dissolving therein a portion or all amount of the compounds of the formula (II) or the anhydrides thereof. The solvent is preferably selected from ethyl alcohol, ethyleneglycol, trimethyleneglycol, tetramethylenglycol, benzene and xylene.
There is no specific limitation to the reaction molar ratio of the titanium compounds (1) with the compounds of the formula (II) or anhydrides thereof. However, if the proportion of the titanium compound (1) is too high, the resultant polyester may exhibit an unsatisfactory color tone and too low a softening temperature. If the proportion of the titanium compound (1) is too low, the resultant catalyst may exhibit an insufficient catalytic activity for promoting the polymerization reaction.
Accordingly, the reaction molar ratio of the titanium compounds (1) to the compounds of the formula (II) or the anhydrides thereof is preferably 2/1 to 2/5. The reaction product of the above-mentioned reaction may be subjected to the reaction procedure with the above-mentioned phosphorus compound (3) without refining or after refining the reaction product by a recrystallization thereof with a recrystallizing agent, for example, acetone, methyl alcohol and/or ethyl acetate.
In the phosphorus compound (3) represented by the general formula (III) usable for the phosphorus compound component (B), the C6-C20 aryl groups or C1-C20 alkyl groups represented by R2 may have no substituent or one or more substituents. The substituents include, for example, a carboxyl group, alkyl groups, a hydroxyl group and an amino group.
The phosphorus compounds (3) of the general formula (III) are selected from the groups consisting of, for example, phenylphosphonic acid, methylphosphonic acid, ethylphosphonic acid, propylphosphonic acid, isopropylphosphonic acid, butylphosphonic acid, tolylphosphonic acid, xylylphosphonic acid, biphenylphosphonic acid, naphthylphosphonic acid, anthorylphophonic acid, 2-carboxylphenylphosphonic acid, 3-carboxyphenylphosphonic acid, 4-carboxyphenylphosphonic acid, 2,3-dicarboxyphenylphosphonic acid, 2,4-dicarboxyphenylphosphonic acid, 2,5-dicarboxyphenylphosphonic acid, 2,6-dicarboxyphenylphosphonic acid, 3,4-dicaiboxyphenylphosphonic acid, 3,5-dicarboxyphenylphosphonic acid, 2,3,4-tricarboxyphenylphosphonic acid, 2,3,5-tricarboxyphenylphosphonic acid, 2,3,6-tricarboxyphenylphosphonic acid, 2,4,5-tricarboxyphenylphosphonic acid, 2,4,6-tricarboxyphenylphosphonic acid, phenylphosphinic acid, methylphosphinic acid, ethylphosphinic acid, propylphosphinic acid, isopropylphosphinic acid, butylphosphinic acid, tolylphosphinic acid, xylylphosphinic acid, biphenylphosphinic acid, diphenylphosphinic acid, dimethylphosphinic acid, diethylphosphinic acid, dipropylphosphinic acid, diisopropylphosphinic acid, dibuatylphosphinic acid, ditolylphosphinic acid, dixylylphosphinic acid, dibiphenylphosphinic acid, naphthylphosphinic acid, anthrilphosphinic acid, 2-carboxyphenylphosphinic acid, 3-carboxyphenylphosphinic acid, 4-carboxyphenylphosphinic acid, 2,3-dicarboxyphenylphosphinic acid, 2,4-dicarboxyphenylphosphinic acid, 2,5-dicarboxyphenylphosphinic acid, 2,6-dicarboxyphenylphosphinic acid, 3,4-dicarboxyphenylphosphinic acid, 3,5-dicarboxyphenylphosphinic acid, 2,3,4-tricarboxyphenylphosphinic acid, 2,3,5-tricarboxyphenylphosphinic acid, 2,3,6-tricarboxyphenylphosphinic acid, 2,4,5-tricarboxyphenylphosphinic acid, 2,4,6-tricarboxyphenylphosphinic acid, bis(2-carboxyphenyl)phosphinic acid, bis(3-carboxyphenyl)phosphinic acid, bis(4-carboxyphenyl)phosphinic acid, bis(2,3-dicarboxyphenyl)phosphinic acid, bis(2,4-dicarboxyphenyl)phosphinic acid, bi(2,5-dicarboxyphenyl)phosphinic acid, bis(2,6-dicarboxyphenyl)phosphinic acid, bis(3,4-dicarboxyphenyl)phosphinic acid, bis(3,5-dicarboxyphenyl)phosphinic acid, bis(2,3,4-tricarboxyphenyl)phosphinic acid, bis(2,3,5-tricarboxyphenyl)phosphinic acid, bis(2,3,6-tricarboxyphenyl)phosphinic acid, bis(2,4,5-tricarboxyphenyl)phosphinic acid, and bis(2,4,6-tricarboxyphenyl)phosphinic acid.
The preparation of the catalyst from the titanium compound component (A) and the phosphorus compound component (B) is carried out, for example, by mixing the component (B) comprising at least one phosphorus compound (3) of the formula (III) into a solvent to dissolve a portion or all amount of the phosphorus compound component (B) in the solvent; adding dropwise the titanium compound component (A) into the component (B) mixture; and heating the resultant reaction system at a temperature of 0 to 200xc2x0 C. for a reaction time of 30 minutes or more, preferably 60 to 150xc2x0 C. for a reaction time of 40 to 90 minutes. In this reaction, there is no specific limitation to the reaction pressure. The reaction may be carried out under pressure (0.1 to 0.5 MPa), or the ambient atmospheric pressure, or a reduced pressure (0.001 to 0.1 Pa), but is usually carried out under the ambient atmospheric pressure.
The solvent usable for the component (A) comprising the phosphorus compound (3) of the formula (III) for the catalyst preparation is not limited to specific solvents as long as the solvent can dissolve therein at least a portion of the phosphorus compound component (B). Preferably, the solvent comprises at least one member selected from, for example, ethyl alcohol, ethyleneglycol, trimethyleneglycol, tetramethyleneglycol, benzene and xylene. Particularly, the solvent comprises a glycol compound the same as the glycol component from which the target polyester is prepared.
In the catalyst-preparation reaction as mentioned above, the mixing ratio of the titanium compound component (A) and the phosphorus compound component (B) in the reaction system is established so that in the reaction product produced from the titanium compound component (A) and the phosphorus compound component (B) and contained in the resultant catalyst, the ratio mTi/mp of the amount of the titanium compound component (A) in terms of a molar amount mTi of titanium atoms contained in the component (A) to the amount of the phosphorus compound component (B) in terms of a molar amount mp of the phosphorus atoms contained in the component (B) is in the range of from 1:1 to 1:4. A preferable reaction molar ratio mTi/mp is in the range of 1:1 to 1:3.
The reaction product of the titanium compound component (A) with the phosphorus compound component (B) is isolated from the reaction system thereof by way of, for example, a centrifugal sedimentation treatment or, filtration and the isolated reaction product may be employed, without refining it, as a catalyst for the production of the polyester, or the isolated reaction product may be refined by, for example, recrystallizing the reaction product from a recrystallizing agent, for example, acetone, methyl alcohol and/or water, and the refined product may be employed as a catalyst.
In the process for producing the polyester of present invention, a polymerization-starting material comprising at least one member selected from alkyleneglycol esters of aromatic difunctional carboxylic acids and polymers having a low degree of polymerization (oligomers) thereof, is polymerized in the presence of the above-mentioned catalyst.
In the process of the present invention for producing the polyester, the amount of the catalyst in terms of molar amount in millimoles of titanium atoms contained therein is established at 10 to 40% based on the total molar amount in millimoles of the aromatic difunctional carboxylic acids contained in the polymerization-starting material. The catalyst amount is preferably 10 to 25% on the same basis as mentioned above. If the catalyst is employed in an amount of less than 10%, the effect of the catalyst for promoting the polymerization reaction of the polymerization-starting material is insufficient, thus the production efficiency for the polyester is insufficient and thus a polyester having a target degree of polymerization is not obtained. Also, if the catalyst is employed in an amount of more than 40%, the resultant polyester has an unsatisfactory color tone, (b value) and is slightly colored yellowish and thus exhibits an unsatisfactory utilizability in practice.
In the alkyleneglycol esters of the aromatic difunctional carboxylic acids usable as a polymerization-starting material for the process of the present invention for producing the polyester, the aromatic difunctional carboxylic acids are preferably selected form terephthalic acid, isophtalic acid, naphthalenedicarboxylic acid, biphenyldicarboxylic acid, diphenylsulfonedicarboxylic acid, diphenyletherdicarboxylic acid, diphenyletherdicarboxylic acid, diphenoxyethanedicarboxylic acid, and xcex2-hydroethoxybenzoic acid, more preferably terephthalic acid and naphthalenedicarboxylic acid. Also, the alkyleneglycols are preferably selected from ethyleneglycol, trimethyleneglycol, tetramethyleneglycol, neopentylglycol and hexamethyleneglycol.
The alkeleneglycol esters of the aromatic difunctional carboxylic acids and/or oligomers may be produced by any process. Usually, the they are produced by heat-reacting an aromatic difunctional carboxylic acid or an ester-producing derivative thereof with an alkyleneglycol or an ester-producing derivative thereof.
For example, ethyleneglycol ester of terephthalic acid and oligomers thereof which are used as a starting material for polyethylene terephthalate will be explained below. They are usually produced by directly esterifying terephthalic acid with ethyleneglycol, or by interesterifying a lower alkyl ester of terephthalic acid with ethyleneglycol or by addition-reacting terephthalic acid with ethylene oxide.
Also, trimethyleneglycol ester of terephthalic acid and/or oligomers thereof which are used as a starting material for polytrimethylene terephthalate will be explained below. They are usually produced by directly esterifying terephthalic acid with trimethyleneglycol, or by interesterifying a lower alkylester of terephthalic acid with trimethyleneglycol, or by addition reacting terephthalic acid with trimethyleneoxide.
The above-mentioned alkyleneglycol esters of aromatic difunctional carboxylic acids and/or oligomers thereof, may be employed together with other difunctional carboxylic esters copolymerizable with the above-mentioned esters or oligomers, as an additional component, in an amount of 10 molar % or less, preferably 5 molar % or less, based on the total molar amount of the acid component, as long as the additional component does not cause the effect of the process of the present invention to be substantially degraded.
The preferably copolymerizable additional component may be selected from esters of an acid component comprising at least one member selected from, for example, aliphalic and cycloaliphatic difunctional dicarboxylic acids, for example, adipic acid, sebacic acid and 1,4-cyclohexanedicarboxylic acid, and hydroxycarboxylic acids, for example, -xcex2-hydroxyethoxybenzoic acid and p-hydroxybenzoic acid with a glycol component comprising at least one member selected from aliphatic cycloaliphatic and aromatic diol compounds, for example, alkyleneglycols having 2 or more carbon atoms, 1,4-cyclohexanedimethanol, neopentylglycol, bisphenol A and bisphenol S, and polyoxyalkyleneglycols, or anhydrides of the esters. The additional components may be employed alone or in a mixture of two or more thereof. The content of the additional components in the copolymers must be in the above-mentioned range.
In the process of the present invention for producing the polyester, a procedure in which the catalyst is added into the polymerization-starting material may be effected in any stage before the start of the polymerization reaction of an aromatic difunctional carboxylic acid-alkyleneglycol ester or an oligomer thereof, and the catalyst-adding procedure may be carried out by any conventional method. For example, after a preparation of an aromatic difunctional carboxylic ester is completed, a solution or slurry of a catalyst may be added into the preparation system, to start a polymerization reaction, or the catalyst may be added into the reaction system, before the preparation of the aromatic difunctional carboxylic ester, together with the starting material or after the starting material is fed into the reaction system.
In the process of the present invention, there is no specific limitation to the conditions for the production of the polyester. Preferably, the polymerization reaction is carried out at a temperature of 230 to 320xc2x0 C. or under the ambient atmospheric pressure or a reduced pressure, for example, 0.1 Pa to 0.1 Mpa, or under the combined conditions of the above-mentioned temperature with the above-mentioned pressure, for 15 to 300 minutes.
In the process of the present invention, the reaction system is optionally added with a stabilizing agent, for example, trimethyl phosphate and further with another additive, for example, antioxidant, ultra-violet ray-absorbing agent, flame-retardant, fluorescent brightening agent, delustering agent, color tone-regulating agent and anti-foaming agent at any stage in the polyester production procedure.
Further, to finely regulate the color of the polyester, during the polyester production procedure, a color tone-regulating agent comprising at least one member selected from organic blue-coloring pigments, for example, ago, triphenylenethane, guinoline, anthraquinone and phthalocyanine blue-coloring pigments, and inorganic blue-coloring pigments may be mixed into the reaction system. Also, in the process of the present invention for producing the polyester, a conventional cobalt-containing inorganic blue-coloring pigment, which causes the melting heat stability of polyester to be decreased, is not necessary as a color tone regulating agent to the polyester, and thus the resultant polyester of the present invention is preferably substantially free from cobalt.
The polyester obtained by the process of the present invention usually exhibits an L value of 80.0 or more and a b value of xe2x88x922.0 to 5.0, determined by a HUNTER color difference meter. If the L value is less than 80.0, the resultant polyester may exhibit too low a whiteness and thus may not form a shaped polyester article having a high whiteness and useful in practical use. Also, if the b value is less than xe2x88x922.0, the resultant polyester may have too high a blueing effect whereas the yellowing effect of the polyester may be low. Further, if the b value is more than 5.0, the resultant polyester may exhibit too high a yellowing effect and thus may not be subjected to the production of a practically useful shaped polyester article.
In the polyester produced by the process of the present invention, the L value is preferably 82 or more, more preferably 83 or more, and the b value is preferably xe2x88x921.0 to 4.5, more preferably 0.0 to 4.0.
The L value and b value of the polyester obtained by the process of the present invention is determined by the following method.
Namely, a sample of the polyester is melted at a temperature of 290xc2x0 C. under vacuum for 10 minutes, the polyester melt is formed into a plate having a thickness of 3.0xc2x11.0 mm on an aluminum plate and, immediately after the forming, the polyester plate is cooled in ice water, the cooled plate is dried at 160xc2x0 C. for one hour, the dried plate is subjected to a crystallization treatment, the resultant plate is placed on a plate having a standard whiteness and for providing a color difference meter ready for use and the color tone of the surface of the plate was measured by a HUNTER color difference meter (model: CR-200, made by MINOLTA CO.).
The polyester produced by the process of the present invention contains substantially no cobalt atoms derived from a color tone-regulating cobalt compound. The cobalt atom-containing polyester is disadvantageous in that polyester exhibits a low melting heat stability and is easily decomposed. The phrase xe2x80x9ca polyester containing substantially no cobalt atomsxe2x80x9d refers to a polyester produced without employing a cobalt compound as a color tone-regulating agent or a polymerization catalyst and thus containing no cobalt atoms derived from the above-mentioned cobalt compound. Therefore, the polyester of the present invention may contain cobalt atoms derived from a cobalt compound added to the polyester for a purpose other than the purposes of the color tone-regulating agent and the catalyst.
The polyester of the present invention preferably has a content of solid foreign particles having an average particle size of 3 xcexcm or more limited to 500 particles/g or less. When the foreign particle content is limited to 500 particles/g or less, blockage of a filter for the melt-shaping procedure and an increase in back pressure in the melt-spinning procedure are significantly restricted. The content of the foreign particles is more preferably 450 particles/g or less, still more preferably 400 particles/g or less.
In the polyester of the present invention, when heated in a nitrogen gas atmosphere at a temperature of 290xc2x0 C. for 15 minutes, preferably the number of breakage of backbone chains of the polyester molecules is limited to 4.0 equivalents/1,000 kg or less. When the breakage number of the backbone chains of the polyester molecules is 4.0 equivalents/1,000 kg or less, the deterioration of the polyester during the melt-shaping procedures is significantly restricted and thus the mechanical properties and the color tone of the shaped article can be improved. The above-mentioned breakage number of the backbone chains of the polyester molecules is more preferably 3.8 equivalents/1,000 kg or less, still more preferably 3.5 equivalents/1,000 kg or less.
In the polyester of the present invention, there is no specific limitation to the intrinsic viscosity thereof. Usually, the intrinsic viscosity of the polyester is preferably in the range of from 0.55 to 1.0. When the intrinsic viscosity is in the above-mentioned range, the melt-processing can be easily carried out and the resultant shaped article exhibits a high mechanical strength. The above-mentioned intrinsic viscosity of the polyester is more preferably in the range of from 0.60 to 0.90, still more preferably from 0.62 to 0.80.
The intrinsic viscosity of the polyester is determined in a solution in orthochlorophenol at a temperature of 35xc2x0 C.